U.S. Pat. No. 4,614,165 granted Sept. 30, 1986 to Folkins et al, is representative of a number of disclosures addressing the problem of the useful life of two-component electrostatographic developers. Two-component developers include finely divided toner particles that are to be deposited on the electrostatic image and carrier particles which may be magnetic. The carrier particles triboelectrically charge the toner particles and also help transport them through a development station. After substantial use, the carrier particles lose their ability to impart the correct charge to the toner particles due to formation of a scum on the carrier particles from mixing with toner and other reasons. They are replaced periodically in most machines.
U.S. Pat. No. 4,614,165 describes a number of methods and apparatus, known per se, that continuously replace the carrier particles. It suggests replacing used toner by supplying to the developing station a mixture of 25% carrier and 75% toner particles by weight. This mixture is added to a developer sump in the station that may be operating with a mixture of 96% carrier and 4% toner particles by weight. The added carrier particles cause the developer in the station to overflow into a separate container that can be emptied by the serviceman.
The above reference and the references discussed therein involve fairly elaborate mechanisms for mixing toner and carrier in the apparatus itself and generally require a serviceman to dispose of the spent developer that overflows.